Add -DCOLVARS_LEPTON, track user's choice via DISABLE_CXX11_REQUIREMENT

  file(GLOB COLVARS_SOURCES ${COLVARS_SOURCE_DIR}/[^.]*.cpp)

  if(DEFINED CMAKE_CXX_STANDARD)

    # The condition below should match most modern C++ standards

    if((${CMAKE_CXX_STANDARD} GREATER 0) AND (${CMAKE_CXX_STANDARD} LESS 70))

      set(ENABLE_LEPTON ON)

  # Build Lepton by default

  set(COLVARS_LEPTON_DEFAULT ON)

  # but not if C++11 is disabled per user request

  if(DEFINED DISABLE_CXX11_REQUIREMENT)

    if(DISABLE_CXX11_REQUIREMENT)

      set(COLVARS_LEPTON_DEFAULT OFF)

    endif()

  endif()

  option(COLVARS_LEPTON "Build and link the Lepton library" ${COLVARS_LEPTON_DEFAULT})

  # Verify that the user's choice is consistent

  if(DEFINED DISABLE_CXX11_REQUIREMENT)

    if((DISABLE_CXX11_REQUIREMENT) AND (COLVARS_LEPTON))

      message(FATAL_ERROR "Building the Lepton library requires C++11 or later.")

    endif()

  endif()

  if(COLVARS_LEPTON)

    set(LEPTON_DIR ${LAMMPS_LIB_SOURCE_DIR}/colvars/lepton)

    file(GLOB LEPTON_SOURCES ${LEPTON_DIR}/src/[^.]*.cpp)

    add_library(lepton STATIC ${LEPTON_SOURCES})

    target_include_directories(lepton PRIVATE ${LEPTON_DIR}/include)

  endif()

  endif()

  add_library(colvars STATIC ${COLVARS_SOURCES})

  target_include_directories(colvars PUBLIC ${LAMMPS_LIB_SOURCE_DIR}/colvars)

  list(APPEND LAMMPS_LINK_LIBS colvars)

  if(ENABLE_LEPTON)

  if(COLVARS_LEPTON)

    list(APPEND LAMMPS_LINK_LIBS lepton)

    target_compile_options(colvars PRIVATE -DLEPTON)

    target_include_directories(colvars PUBLIC ${LEPTON_DIR}/include)